This invention relates to chemical reductive plating or electroless plating of nickel or cobalt. More particularly, it relates to such plating onto surfaces of materials which are normally noncatalytic to the plating bath. Such materials include copper; chromium-containing stainless steels; Kovar alloy of iron, nickel and cobalt; silver; electrically conductive molymanganese and tin oxide-antimony oxide compositions; and other materials.
The chemical deposition of nickel or cobalt onto such surfaces can be accomplished by contacting the surface with an active metal, such as steel or aluminum, or by impressing a small cathodic potential, or by prior treatment in acid palladium chloride solution. These methods have disadvantages especially if the metal surfaces exist as discrete isolated areas, such as on printed circuit boards.
The use of steel shot rolled or tumbled over the surface can activate isolated areas, but as the number of areas per board increases and the areas become smaller, larger amounts of shot are required, and the proportion of useful coating decreases. It is not unusual to use and plate 20 square feet of shot to plate 0.2 square foot of circuit pattern in 100% yield. Also, steel racks can provide the local catalytic activity needed to commence plating on a continuous sheet of a metal such as copper. Of course, the rack would not normally touch all the areas to be plated on a circuit board, and so, this approach would not be effective for such plating.
Application of an electrical potential also activates these surfaces but requires special care in design and application to insure 100% activation and is difficult to use for discontinuous patterns, such as in the plating of conductors on circuit boards. Palladium treatment generally activates these surfaces, but it also activates ceramic or plastic surfaces on which metal is deposited, and this degrades quality with shorting between conductor pads.
Electroless or chemical reductive plating of nickel and cobalt is well known and described in the literature and in many patents. Among the informative patents are the following which are hereby incorporated by reference:
U.S. Pat. No. 3,096,182, granted July 2, 1963, to Berzins, describes the use of alkali metal borohydrides as the reducing agents in chemical plating baths. U.S. Pat. No. 3,338,726, granted Aug. 29, 1967 to Berzins, concerns the use of amine boranes as the reducing agents. U.S. Pat. No. 3,738,849 granted June 12, 1973, to Bellis is concerned with the use of alkali metal cyanoborohydrides in electroless plating. Each of these patents is assigned to the assignee of the present invention. U.S. Pat. No. 3,295,999 granted Jan. 3, 1967, to Klein and Zirngiebl also gives further background on these materials. These patents utilize boron compounds as the reducing agent, leading to the incorporation of boron in the nickel or cobalt coating, as described in U.S. Pat. No. 3,045,334, granted July 24, 1962 to Berzins and U.S. Pat. No. 3,674,447, granted July 4, 1972 to Bellis (including thallium in the coating), both of which are assigned to the assignee of the present invention.
The use of hypophosphite reducing agents is also well known. A discussion of this technology is presented in "Modern Electroplating", ed. F. A. Lowenheim, pages 699 to 708, published by John Wiley & Sons, Inc., 1968.
Dimethylamine borane and sodium hypophosphite solutions have been used to maintain prepared aluminum surfaces after appropriate treatments and prior to nickel or cobalt chemical plating, as described in U.S. Pat. No. 3,667,991--Miller, issued June 6, 1972. However, aluminum is normally catalytic to chemical plating baths, and the function of these solutions is to minimize oxide formation and permit a more uniform and adherent coating to be formed. In that application, dimethylamine borane and sodium hypophosphite are considered to be equivalent, and aluminum is receptive to nickel or cobalt electroless plating without exposure to these solutions.